Field
The present disclosure relates generally to the field of lithium-ion batteries, and more particularly, to a lithium-ion battery bonding agent and a lithium-ion battery comprising said bonding agent.
Background
With advantages such as high energy density, high working voltage, and long cycle life, lithium-ion batteries have been extensively used on civilian apparatuses such as mobile phones, laptops, and digital products, as well as high-tech equipment such as unmanned aerial vehicles, space flight, and satellites. Lithium-ion batteries mainly comprise a positive film, a negative film, a separator and an electrolyte. The negative film typically uses natural graphite or artificial graphite as an active substance, sodium carboxymethyl cellulose (CMC) as a dispersing agent, and styrene-butadiene rubber (SBR) as a bonding agent. In the positive film, on the other hand, lithium cobalt oxide (LiCoO2) is commonly used as an active substance, conductive carbon as a conductive agent, and polyvinylidene difluoride (PVdF) as a bonding agent.
When fabricating a lithium-ion battery, firstly, coat positive and negative electrode pastes on corresponding current collectors, dry and then subject to cold pressing, and then perform subsequent processes. After cold pressing, graphite, CMC, and SBR in the negative electrode (anode) are all soft materials themselves, thus the electrode film is relatively soft. As for the positive electrode (cathode), however, LiCoO2 and PVdF have relatively high hardness, and PVdF has a strong crystallinity, making it easy to have the issue that the electrode film becomes brittle after cold pressing, and may lead to problems that the active substance cracks and falls off in the subsequent strip division and sheet cutting, and that the electrode film breaks in the winding process, which detrimentally affect the battery production and performance.
To improve the flexibility of a positive film after cold pressing, a method of modification by copolymerization is typically adopted on PVdF. Homopolymer PVdF has a strong crystallinity, and the material itself has high strength and hardness, and the introduction of other structural units into PVdF by means of modification by copolymerization may reduce the degree of order of the PVdF molecular structure, lower crystallinity, and may improve the ductility and elongation at break of PVdF. When the modified PVdF is used as a bonding agent for a positive film, the flexibility of the electrode film after cold pressing can be improved.
For performing modification by copolymerization on PVdF, a commonly used copolymerization monomer is hexafluoropropylene, and the obtained PVdF-HFP has a significantly lowered crystallinity and improved elongation at break relative to homopolymer PVdF. When PVdF-HFP is used as a bonding agent for a positive film, the flexibility of the electrode film is improved, which helps to improve the compact density, thus improving battery energy density. However, the bonding force of PVdF-HFP is significantly reduced relative to copolymer PVdF. During the manufacture process, the positive electrode active substance could fall off as a result of the low bonding force. When an electrolyte is injected into the battery, PVdF-HFP swells greater than that of homopolymer PVdF does, the bonding force of the positive film decreases by a greater degree, leading to the problem of the active substance falling off after formation and aging and during the battery use, and consequently leading to significantly decreased performance or failure of the battery.
The present disclosure is hereby proposed in view of the drawbacks of the traditional technologies.